Field of the Invention
The invention pertains to a system of digital imaging with grey scale setting, especially for the display of blood vessels.
In a digital imaging installation an image, most often an X-ray type of image, comprises points each of which has a digital value assigned to it, representing a radiation intensity. The digital value has a precision which is that of the measuring instruments. This precision is generally such that the said digital value can be represented by a binary number with 10, 11 or 12 bits.
Given that, in daytime (i.e. under the optimum conditions), the human eye is capable of distinguishing only about 1,000 different half-shades between dazzling white and absolute black provided, moreover, that these half-shades are located contiguously, a precision of 0.1% in the luminance is sufficient. But a cathode-ray tube can restore only 100 half-shades. Thus the luminance value assigned to each point during image reproduction is generally a binary number with a maximum of 8 bits (256 values possible) while the measuring instruments allow 10 to 12 bits.
In the reproduction of an image on a television screen, to benefit from all the precision of the digital image, an operation generally known as "window-making" is performed: in this operation, only brightness signal values between a minimum and a maximum are selected, the values between these limits being called a "window". The values of the digital signal which are outside the window correspond, on the one hand, to black (generally for values smaller than the lower limit) and on the other hand, to white (generally for values exceeding the upper limit of the window).
The window can usually be modified at will by the user depending on the object observed and on what he is seeking in this object. For example, to observe a human tissue by digital X-ray photography, one limit of the window corresponds to the highest absorption levels due to the bone while the other limit corresponds to the lowest absorption levels through the ambient air.
A window is defined by two parameters, most usually the width L and the mean level M. The width L of the window is the difference between the two limits while the mean level M is a value between these two limits which gives a value of grey determined on the display screen. The width L influences the contrast while the mean level M represents the luminosity of the image.
Window-arranging can be done by means of a random-access memory, called an equivalence table, receiving at its addressing input a signal representing the digital value of the radiation intensity at a point of the image and the content of the memory box which is at the corresponding address is transmitted to a television screen through a digital-analog converter. To modify the window, there is provision for computing means which modify the content of each box of the equivalence table according to the values of the two parameters which characterize the window. Hence, for each parameter, a setting means, such as a potentiometer, is associated with the computing means.
But the actuating of two commands is a constraint which users often consider to be a handicap.